Studies will be initiated to examine the factors, myocardial or valvular, which are responsible for generation of the first heart sound. These factors will be assessed in the dog under conditions of right heart bypass with simultaneous assessment of hemodynamics, angiocardiography and echocardiography. As in previous studies of this type we attach radiopaque metallic clips to leaflets of the mitral valve to accurately delineate mitral valve motion. It has been previously assumed that the echocardiograph accurately defines mitral valve closure with the implication that the active mitral valve closure generates the first heart sound. Under the methodologic condition described it should be possible to determine the exact structural basis for generation of the first heart sound and its precise relationship to mitral valve closure. These studies appear to have significant import for the understanding of normal mitral valve function and for the non-invasive diagnosis of mitral valve disease.